Question: Which of these numbers is prime? ${25,\ 26,\ 35,\ 39,\ 43}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 25 are 1, 5, and 25. The factors of 26 are 1, 2, 13, and 26. The factors of 35 are 1, 5, 7, and 35. The factors of 39 are 1, 3, 13, and 39. The factors of 43 are 1 and 43. Thus, 43 is a prime number.